From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8A6F8C433DF for ; Mon, 24 Aug 2020 16:28:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 54DD9214F1 for ; Mon, 24 Aug 2020 16:28:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726631AbgHXQ2L (ORCPT ); Mon, 24 Aug 2020 12:28:11 -0400 Received: from mx2.suse.de ([195.135.220.15]:47036 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727896AbgHXQ1z (ORCPT ); Mon, 24 Aug 2020 12:27:55 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 31DDFAF76; Mon, 24 Aug 2020 16:28:17 +0000 (UTC) Subject: Re: [PATCH] x86/entry: Fix AC assertion To: Andrew Cooper , peterz@infradead.org Cc: x86@kernel.org, LKML , Andy Lutomirski References: <20200824101428.GS1362448@hirez.programming.kicks-ass.net> <20200824152135.GV1362448@hirez.programming.kicks-ass.net> From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= Message-ID: Date: Mon, 24 Aug 2020 18:27:46 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 24.08.20 17:58, Andrew Cooper wrote: > On 24/08/2020 16:21, peterz@infradead.org wrote: >> On Mon, Aug 24, 2020 at 03:22:06PM +0100, Andrew Cooper wrote: >>> On 24/08/2020 11:14, peterz@infradead.org wrote: >>>> The WARN added in commit 3c73b81a9164 ("x86/entry, selftests: Further >>>> improve user entry sanity checks") unconditionally triggers on my IVB >>>> machine because it does not support SMAP. >>>> >>>> For !SMAP hardware we patch out CLAC/STAC instructions and thus if >>>> userspace sets AC, we'll still have it set after entry. >>> Technically, you don't patch in, rather than patch out. >> True. >> >>>> Fixes: 3c73b81a9164 ("x86/entry, selftests: Further improve user entry sanity checks") >>>> Signed-off-by: Peter Zijlstra (Intel) >>>> Acked-by: Andy Lutomirski >>>> --- >>>> arch/x86/include/asm/entry-common.h | 11 +++++++++-- >>>> 1 file changed, 9 insertions(+), 2 deletions(-) >>>> >>>> --- a/arch/x86/include/asm/entry-common.h >>>> +++ b/arch/x86/include/asm/entry-common.h >>>> @@ -18,8 +18,15 @@ static __always_inline void arch_check_u >>>> * state, not the interrupt state as imagined by Xen. >>>> */ >>>> unsigned long flags = native_save_fl(); >>>> - WARN_ON_ONCE(flags & (X86_EFLAGS_AC | X86_EFLAGS_DF | >>>> - X86_EFLAGS_NT)); >>>> + unsigned long mask = X86_EFLAGS_DF | X86_EFLAGS_NT; >>>> + >>>> + /* >>>> + * For !SMAP hardware we patch out CLAC on entry. >>>> + */ >>>> + if (boot_cpu_has(X86_FEATURE_SMAP)) >>>> + mask |= X86_EFLAGS_AC; >>> The Xen PV ABI clears AC on entry for 64bit guests, because Linux is >>> actually running in Ring 3, and therefore susceptible to #AC's which >>> wouldn't occur natively. >> So do you then want it to be something like: >> >> if (boot_cpu_has(X86_FEATURE_SMAP) || >> (IS_ENABLED(CONFIG_64_BIT) && boot_cpu_has(X86_FEATURE_XENPV))) >> >> ? Or are you fine with the proposed? > > Dealers choice, but this option would be slightly better overall. > > (Are there any other cases where Linux will be running in Ring 3?  I > haven't been paying attention to recent changes in PVOps.) I'm not aware of any other case running kernel code in ring 3. Juergen